tv [untitled] March 23, 2011 10:30pm-11:00pm PDT
... >> i am the chief building inspector. we are here at the base of telegraph hill to talk about a subject of great interest for the people of san francisco which is rockslide, slope stability, which caused a dramatic front-page news. i am here with two knowledgeable and wonderful guests. i am here with a geotechnical engineer and a geologist. we are here to talk about rockslide, rockfalls, and related issues. what is the difference between a
geologist and a geotechnical engineer? >> and engineering geologist deals with identifying site characteristics, mapping, the ground's surface, collecting all of this data. the engineer can come up with medications and designs. >> the structural engineer might be working on a building if there is a building involved. what is your role? >> i am interested in the physical properties of the earth and how the earth will behave: subject to different load changes. when you are building a building, you are applying a load and you are wondering if the earth will be able to supply
that load. we work closely together and we come up with practical engineering solutions. >> this is when someone wants to build something. also, we have a rock piece of land. we have to have a resolution. >> in the u.s., about 2/3 of the population lives in areas that are prone to landslides. about $2 billion of damage occurs annually from landslides. unfortunately, 20-25 million people die as a result of landslides o. >> much of the coastline is either a bright red or a beige print th. >> here we are at the base of
telegraph hill on lombard street. this is owned by the city. behind you is a large piece of something exposed. you are looking at a large class that was xextricated in a quarry about hundred years ago. this is a secretive sandstones, shales, accumulated debris. essentially it ended up piled up here. the quarry activity was so intense and they used some much at dynamite that the kind of over blasted. 10 feet of the face was left shattered. you can see the fresh colors and a pile of debris which is precariously perched on the edge of a cliff up there. it is more fresh and more
recent than the rest. it stands out because there's no vegetation. there is no weathering of material. those are the kinds of things you look for. >> there are many types of slides. there is the bedrock slides which we are looking at in is a combination of a block slide and where the material is just out of balance and is falling off. rainfall pushes from the ground water and out of the cliff. as you can see, the slides artistic one of many types of landslides or slope failures. >> do we have all of these kind of failures in san francisco? >> unfortunately, we have them all. the rock from here is used from the sea wall. it came from this hill.
it was a itgnoll that was present. because of this blasting that occurred, the rock is weak. while most of the time, there are times of heavy rain. sometimes, this has nothing to do with the rainfall. iraq is 50 to an old and then it just falls. in this particular area where we're standing, we have seen the wall at the base. >> the big steel beams with the fence. this is an effective way of preventing the rock that does fall from causing damage. that might protect the bottom of the hill but it doesn't do anything over the hill.
>> it has long been my understanding that the city of san francisco says and the building department takes this position that there is no one buildable lot -- unbuildable lot. i wonder if this is realistic. >> if money is an issue, we can develop solutions that will mitigate the impact. that a solution can be eliminating the hill, that can eliminate the problem. or build a structure right into the hill so there is no longer an exposed face or bold enough of iraq together so that it acts as a big buttress to prevent further movement. -- or hold enough of the rock
together so that it acts as a big buttress. we are allowing the wall of the building to act as a retaining structure. there was a time when we did not get any. there was an epic one before in 1982. we brought with us and rainfall charge. >> you were saying that this is somehow correlated to rainfall. this goes from 1914 until 2006. it is the red line in the backgrounds which are the annual rainfall from each year is plotted on here. you can see it varies all over the place. if you take an average, the average is a thin gray line. that does not give you a feel for what the rainfall really is. that is an average over four
years. you can see that these build up until we had the big events before. >> you expect to see them during the apocrine fall. in fact, they might occur at that time but they may occur the year after and two years after because it has taken awhile for it to accumulate. this triggered some major landslides that we saw. these are portions of the cliff rotates out to sea. >> when we look at this cliff, can you tell us what of the features that we can see? >> we are lucky to have a very fresh exposure. there is a large overhanging block. at the very edge, you can see
the roots coming out. they are part of the problem. in addition, these are sandstone and shale. they have been uplifted and faulted and put into the position they are in today. on the cliff face, you can see the bedding of the shale. there are vertical cracks that filled the water and plans is growing. -- and plants are growing. the cumulative effect of the fresh cut with the debris sitting on the ledge, luckily we have a chance there to protect us from rocks going on the street. >> this is one type of rock salt and we will look at a few others. what will we see? >> i think it would be appropriate.
we have been talking about rock salt. there is instability that occurs at telegraph hill. this was more in the earth flow and debris flow kind of movement. this moves downhill as a result of the rations from the earthquake. that is where we are headed next. >> here we are on the next stop of our landslide for parent though this is the lagoon ouna a area. this is different from where we all were just add on telegraph hill. this is a whole different
concept. tell us what we have to. we have the big hill behind us, what kind of formations do we have? >> this part of san francisco, we are out in the dune sands which makes up the hill to my right. a crusty old drainage that we are standing is bedrock, 160 million year-old sandstone. the project right in the hillside over there and then farther down along the path that goes to the reservoir. >> there are rock slides, there is unconsolidated material. we have an earth float type slide and a debris flows which are common on the these hillsides where you start to see the evidence of creaks when the
trees start to lean over. >> this is a slow movement of the slope. >> yes. you can start to get moving. that will push the trees over a little bit. this had a special history. we had a lot of damage here. i guess the water from leaking laguna runs underground out to golden gate park. it covers about two or three blocks along side to seventh avenue. there was a lot of water damage we might have even had some liquefaction soil failure. we had a major failure. >> that's right. this tribute goes all the way to golden gate.
this is in a mountain lake and beyond ththat, it was a surface flow. this is a reservoir. on my right is this defensive the positive dune sand. the wind blows, it comes from the ocean beach. because the sand is moving, you can see evidence along this side of the hill in 1989. the vibrations in accelerated. you have a significant amount of movement to the point that the homes that uc had very severe damage for two blocks. we recognized what the problem was at the time, it was a matter of loose sand on a very steep slope. thank you for the effort. we were able to get federal funding and design which you
see in the upper walls. the lower walls was actually billed as part of the wpa. the upper wall was built in 1991-'92. this is a concrete wall and it has rocked going through the wall that extends the distance of 80 feet beyond the wall. the next earthquake occurs, the wall is designed to resist and the movement of the sand and allowed it to move down hill. this is a classic example of the creek and degree flow and exists all the way the area. we might see the other evidence. we have done nothing to improve the quality and the strength of the dam. >> one of the things that has come up in the policy issue over and over is that people are
concerned that construction of a new building or the upper story might exacerbate these kinds of soil problems. we have extra review where we might have extra problems. it does that construction have an impact? it is a good idea to come to a peer review or a geotechnical review process as part of determining. >> in january of this year, the california building code requires almost all of san francisco that we get geologist and a geotechnical engineer. it was a state building code rather than just the local requirements. >> as you recall, this was
called the edge hill ordinance. >> is this the same kind of condition? >> this is more of the rock, all on one side and on the other side -- >> this is a combination. >> a lot of sand hills were scrapes down and deposited to other places without being engineered or consolidated and people build on top of that. we have a whole change as the city surfaced. what is the deal with sand, is
this a good place to build on? >> if you want to understand the properties and if you designed to mitigate any adverse effects of the settlement, this is the strongest. i live in the sunset, i have been doing this right on the other side of this hill. i see the same conditions exist here where we have the earthquakes and the sand shakes and it gets more dense. we see differential settlement of are on. >> they might have filled in a low land or a wet land and build on top of it. this is a fully grated sand. seeing difficult to distinguish what was natural. you could be growing like we do and as difficult to distinguish
the context between this and the natural sand. we have to go by its consistency. >> in san francisco, we have a tremendous resource. we have a collection of soyoil supported that goes back many decades and they cover the whole realm of the city. we have them at the apartment building inspections. someone understands what they are. you can look at them and read them and find one in your neighborhood. is that right? >> that is true. many geologists have spent many years mapping the geologic conditions. there are excellent maps of the city. in fact, there are a hazard maps. in 1974, there was a landslide map. >> that's right.
that is available at that apartment building inspection. these are excellent maps. >> there is a real legacy of geologists working and now cannot always published. san francisco has a great repository of some of these archives. there is a child just -- a geologist to publish letters that is a great source of information. we are on the coast, they have the definitive work for all of the [inaudible] >> and did they pick up the old [inaudible] and wetlands? >> there was a map of those that
weren' off the field. you can see where these are with respect to those that have been filled in with a debris from the 06 earthquake. >> that is one of the things i've learned, the very local zones, exactly where you are building at. side-by-side, san francisco is so varied in its tomography and geology, you cannot tell by looking at the site specifically. >> this will be at the end of the drive. you can show which time we will be looking at out there.
>> these are called rotational. >> there can be a lot of movement all at once. they can slowly creep for long amounts of time. >> here we are at the top of an escarpment that pretty much the end of the world. it is windy and cold. just south of here is some also rock where the san andreas fault goes into the pacific ocean. this is closest to the san andreas fault.
many people believe that the closer you are to default, the greater the ground motion might be. that is a portion that is a south westerly portion of san francisco that has some special earthquake hazards. we are here to talk about the geology of this area. why don't you tell us about what is going on? >> there is a giant step here at the edge of the escarpment, the edge of the landslide. all the land to the west of us is part of a landslide that is upwards of 200 feet deep. it goes underneath the ocean. it causes damage and it was mapped by the earliest geologists'. there are many kinds of rocks here. there is a big sequence of merced formation. this is a base in deposit that
was formed by the interaction at the end of the continental margin. >> this is just on top. this is the same material that we were looking at at the last site. you can see it is on our way to the last side. it is deposited here by wind. it is 40-50 feet thick. this acts as a big sponge. the water flows through and it gets in a lower formation and then the water level starts to rise and it becomes a reservoir for water.
this is dissipated during the more dry months of the year. >> this forms a big long skinny basin and runs into san francisco. it is the coastal bluffs areas that are subject to erosion from the waves and the ground water that comes out and you start to lose this. this is the largest example of failure for a long part of the coast. it has been moving continuously for a long time. >> the ocean was at the same level as we are standing. in 1982, and massive slide occurred further to the north of and one day it moves 5 feet and by the second day it moves 30 feet out. all of that land down below was
sitting out here. it was a great deal logic feature of the landslide further to the south. there were houses on top and there was a very steep slope. you can see some debris slides where the sand is slowing. then you drop into a gully or a drainage swell. the top of the landslides' is just a drop in the middle where another clock has pulled away from the edge. the higher hill on the outside edge of the rich and the depression is the drop. surface erosion is an acute issue. >> you can see the evidence of the houses directly adjacent. we have similar conditions to that. if you go around the corner around the seacliff, where in
fact there has been significant movement and in fact many of the homeowners along that portion of the seacliff have had to come back in and do extensive foundation repair and massive walls to protect the home while the slope [inaudible] >> here we have the shallow slides, a debris slides. during earthquakes, they are both activated and the movement on the deep sea slides accelerates as well as the official material that erodes. >> if there's one point that is brought home from this discussion of landslides is san
francisco is a part of the community that experiences many types of landslides, rockfalls, deportations. we need to be aware of the geologic and topographic setting we are in. when we are planning to build our home or buy a home or at additions to the home. if i have one suggestion, that is to seek information from the apartment building inspection. if you're not satisfied, there is ample information there. a geologist needs an opinion of the stability of the environment you're in. >> the building department will typically require that kind of information. people will do a major edition. what we are wondering is why we require that information.